Method and device for recovering a wanted acoustic signal from a composite acoustic signal including interference components

ABSTRACT

The device for extracting a useful acoustic signal from a composite acoustic signal including nuisance components includes: first sensor component for picking up the composite acoustic signal; second sensor component for picking up, in real-time, the reference acoustic signal, which is substantially correlated with the nuisance components of the composite acoustic signal; adaptive filtering component; and extraction component. The coefficients of the filtering component are adapted in real time according to an algorithm chosen so as to minimize the energy of the composite acoustic signal as a function of the energy of the reference acoustic signal, until the signal leaving the output of the extraction component corresponds substantially to the useful acoustic signal selectively rid of the nuisance components.

FIELD OF THE INVENTION

The present invention relates to the extraction of a useful acousticsignal from a composite acoustic signal comprising nuisance components.

It finds an application in the processing of a composite acoustic signalcomprising a useful acoustic signal, for example the voice which it isdesired to record, and nuisance components originating from some sourceof noise (noise from a machine or vehicle, etc.), the said nuisancecomponents being capable of disturbing the recording of the voice.

DESCRIPTION OF RELATED ART

It is already known to cancel nuisance components contained in acomposite acoustic signal.

For example, the Patent GB-A-2273359 describes a device for processing auseful acoustic signal so as to relieve if of its nuisance component.Two geophones pick up the same composite signal consisting of a usefulsignal and of its nuisance components. A delay which depends on thenature of the useful signal and on the distance between the two sensorsis imparted to one of the signals thus picked up and the signals, arenext subtracted in order to extract the useful signal relieved of itsnuisance components.

Such a device is complicated and of limited use on account of the delayto be imparted to the propagation of the signals, geometric constraintsrelated to the siting of the sensors and filtering limitations relatedto the nature of the useful signal to be processed.

It is also known to use characteristics relating to the nuisancecomponents to be canceled (spectral density and bandwidth for example),as well as to the useful acoustic signal to be processed (periodicityand distribution).

However, access to all these characteristics is expensive and difficultto set in place, and may sometimes even be impossible when the nuisancenoise is random, liable to vary over time, in terms of level andfrequency, and/or stretches over a wide band of frequencies and when,additionally, there is no a priori knowledge regarding thecharacteristics of the useful acoustic signal.

SUMMARY OF THE INVENTION

The present invention provides a solution to precisely this problem.

It aims to provide an extractor of a useful acoustic signal from acomposite acoustic signal comprising nuisance components, which may beof any type, even random and broadband, and which are liable to disturbthe processing of the useful acoustic signal in respect of which no apriori knowledge is available.

It pertains to the device for extracting a useful acoustic signal from acomposite acoustic signal comprising nuisance components, including:

first sensor means, which are arranged at a first chosen location andare able to pick up the composite acoustic signal,

second sensor means, which are arranged at a second location chosenaccording to a predetermined geometric relationship with the firstlocation, and are able to pick up, in real time, a reference acousticsignal which is substantially correlated with the nuisance components ofthe composite acoustic signal and is capable of propagating from thesaid second location to the said first location,

filtering means, of the adaptive type, possessing a first input linkedto the second sensor means, a second input and an output, and

extraction means possessing a first input linked to the first sensormeans, a second input linked to the output of the adaptive filteringmeans, and an output linked to the second input of the adaptivefiltering means.

According to a general definition of the invention, the second locationis chosen so that the reference acoustic signal contains no informationrelated to the useful signal and the filtering means are able tominimize the energy of the composite acoustic signal as a function ofthe energy of the reference acoustic signal, by adapting at least someof the filtering coefficients in real time, until the signal leaving theoutput of the extraction means corresponds substantially to the usefulacoustic signal selectively rid of the nuisance components.

The subject of the invention is also a process for extracting a usefulacoustic signal relieved of its nuisance components, implemented by theabovementioned extraction devica, in which the process comprises thefollowing steps:

a) providing first sensor means, which are arranged at a first chosenlocation and are able to pick up the composite acoustic signal,

b) providing second sensor means, which are arranged at a secondlocation chosen according to a predetermined geometric relationship withthe first location, and are able to pick up, in real time, a referenceacoustic signal which is substantially correlated with the nuisancecomponents of the composite acoustic signal and is capable ofpropagating from the said second location to the said first location,

c) providing filtering means, of the adaptive type, possessing a firstinput linked to the second sensor means, a second input and an output,and

d) providing extraction means possessing a first input linked to thefirst sensor means, a second input linked to the output of the filteringmeans, and an output.

According to an important characteristic of the process according to theinvention, the second location is chosen so that the reference signalcontains no information related to the useful signal, and the processcomprises a step consisting in:

e) minimizing the energy of the composite acoustic signal as a functionof the energy of the reference acoustic signal, by adapting at leastsome of the filtering coefficients in real time until the signal leavingthe output of the extraction means corresponds substantially to theuseful acoustic signal selectively rid of the nuisance components.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will emerge in thelight of the detailed description below and of the appended drawings inwhich:

FIG. 1 diagrammatically represents the architecture of the elementsmaking up the device according to the invention, with analog summatormeans;

FIG. 2 is a functional diagram of the elements of FIG. 1;

FIG. 3 is a diagrammatic representation of the architecture of theelements making up the device according to the invention, with digitalsummator means; and

FIG. 4 is a functional diagram of the elements of FIG. 3.

DETAIL OF DESCRIPTION

In FIG. 1, a sensor 2 is arranged at a chosen location so as to pick up,in real time, a composite acoustic signal Mn to be processed. Thiscomposite signal consists of a useful signal Un and nuisance componentsPn. The useful signal Un is for example the voice of a person which itis desired to record. The characteristics of the useful acoustic signaland of the nuisance components are not known a priori. The nuisancecomponents may originate from any source of noise arranged some distancefrom the sensor 2.

The signal Mn picked up by the sensor 2 is here a signal of analog type.

Preferably, the sensor 2 is an omnidirectional microphone, so as to pickup the composite signal.

Of course, any sensor capable of detecting an acoustic signal or itsrepresentation may be applied to the invention.

With reference to FIGS. 1 and 2, the composite signal Mn is applied tothe first positive input 4 of a summator element 6, of the analog type,which will perform the extraction of the useful acoustic signal from thecomposite acoustic signal, as will be seen in greater detail below.

For example, the summator element is an operational amplifier such asthat marketed by the TEXAS INSTRUMENTS company, under the reference TLE2061, organized here so that the signal supplied to these two inputsexhibits substantially the same dynamic level so as to allow optimalextraction.

The summator element 6 comprises a second positive input 8 whichreceives a signal An leaving the output 17 of adaptive filtering means12, via restitution means 32 which will be described in greater detailbelow. This signal An is here likewise analog.

The summator element forms the sum of the analog signals Mn and An and,at its output 10, delivers an analog signal Sn representative of thesaid sum of the signals Mn and An.

The adaptive filtering means 12 comprise a first input 14 receiving thesignal from the second sensor 16. This sensor 16 provides a referenceacoustic signal Rn, which is strongly correlated with the nuisancecomponents Pn to be canceled. The filtering means comprise a secondinput 15 which will be described in greater detail below.

Advantageously, the reference acoustic signal Rn does not containinformation related to the useful signal Un. It is also analog here.

The sensor 16 is arranged at a location chosen according to a particulargeometric relationship with the source of noise (not represented).

For example, the sensor 16 is arranged in proximity to the nuisancenoise source.

The sensor 16 is for example a directional microphone, so as to pick upthe nuisance acoustic signal from the nuisance noise source.

Of course, other transducers may be applied to the picking up of thereference signal. Here, the only constraint is of picking up, in realtime, a reference acoustic signal which can propagate from the locationat which the sensor 16 is arranged to the location at which the sensor 2is arranged.

It is relevant to note here that the designation "reference acousticsignal" does not signify "an acoustic signal emanating from asynchronous reference". Here, the reference acoustic signal is anacoustic signal which is picked up by a microphone and which will serveas a reference for processing the signal. It does not emanate from asynchronous reference. Neither does it correspond to the inverse of thenoise to be attenuated.

It is known that the principle of active attenuation is based on thefact that the speed of propagation of sound is slower than the speed ofpropagation of electricity. In order to comply with this temporal delay,it is expedient to establish a minimum distance separating the sensor 2and the sensor 16. This minimum distance is less than 0.5 m to 2 m fornoise with a broad spectral band or spectral lines.

The essential functions carried out by the adaptive filtering means 12and the analog summator means 6 have been described heretofore withreference to FIG. 2. With reference to FIG. 1, the structure of thedevice according to the invention is supplemented with an acquisitionstage upstream of the adaptive filtering means and with a restitutionstage mounted downstream of the adaptive filtering means.

More precisely, the upstream stage comprises analog/digital acquisitionmeans 18 which receive the reference signal Rn. These acquisition meanscomprise, in series, an input pre-amplifier element (not represented), aconditioning filter (not represented) and an analog/digital converter(not represented).

The pre-amplifier element possesses an input 20 which receives thesignal from the sensor 16, and an output. The conditioning filterpossesses an input linked to the output of the pre-amplifier element,and an output. The analog/digital converter possesses an input linked tothe output of the conditioning filter and an output 22 applied to theinput 14 of a digital signal processor 12 which carries out the adaptivefiltering.

The conditioning filter is advantageously an anti-overlap filter with acutoff frequency chosen according to the application. For example, thecutoff frequency is equal to 2142 Hz for the attenuation of noiseemanating from a compressor (spectral lines).

The processor 12, also termed DSP for DIGITAL SIGNAL PROCESSOR, is forexample that marketed by the TEXAS INSTRUMENTS company under thereference TMS320C25.

The output 17 of the processor is applied to the input 30 of arestitution module 32 which comprises, in series, a digital/analogconverter (not represented) and a smoothing filter (not represented).

For example, the smoothing filter is a filter of the low-pass type whoseinput receives the signal leaving the digital/analog converter and whoseoutput 34 is linked to the second input 8 of the summator means 6.

The output 34 of the digital/analog restitution module delivers thesignal An.

The sampling frequency of the analog/digital and digital/analogconverters is chosen so that it is possible to sample the acousticsignals during the time required for the reference acoustic signal topropagate from the location at which it is picked up to the location atwhich it will perturb the useful acoustic signal.

The cutoff frequency of the conditioning and smoothing filters is chosento be relatively high in the case of random noise, since the delay inthe electronics increases as the value of the cutoff frequencydecreases.

For example, for a cutoff frequency of the order of 2000 Hz and asampling frequency of 5000 Hz, the minimum distance between the sensors2 and 16 is 0.4 m.

For example, for a cutoff frequency of the order of 800 Hz and asampling frequency of 2000 Hz, the minimum distance between the sensors2 and 16 is 1 m.

In the case of muted and low-pitched noise which corresponds tofrequencies below those of the voice, it is possible to chooserelatively low sampling frequencies, that is to say of the order of 2 to5 kHz, with summator means of analog type.

At its second input 15, the processor 12 receives a signal from anotheracquisition module 36 whose input 38 receives a signal from the outputof the summator means 6, that is to say the signal Sn. This secondacquisition module 36 comprises an output 40 applied to the input 15 ofthe processor 12.

In practice, the acquisition module 36 comprises the same elements asthe acquisition module 18, namely, in series, an input pre-amplifier, aconditioning filter and an analog/digital converter.

According to the invention, the processor 12 runs an algorithm forminimizing the energy of the composite acoustic signal Mn as a functionof the energy of the reference acoustic signal Rn, until the acousticsignal Sn leaving the output of the summator means correspondssubstantially to the useful acoustic signal Un selectively rid of thenuisance components Pn.

The minimization algorithm is preferably that referred to as LMS,standing for LEAST MEAN SQUARE.

According to the principle of active attenuation, the signal An mustcorrespond to the amplitude of the signal Pn but be in phase oppositionrelative to it.

The signal An is obtained via the acquisition of the signal Rn and ofthe signal Sn.

The sum of the signals Mn and An makes it possible to deduce the signalUn therefrom.

At the instant t, the processor 12 determines the weighting coefficientsW to minimize the energy of the signal Sn until this energy correspondsto that of the useful signal Un.

For example, the number of weighting coefficients W is 90.

The signal Sn here corresponds to the noise-cleaned composite signal.The latter can be recorded with the aid of an appropriate recorder 50 onan audio tape, or drive a loudspeaker via a power amplifier (these arenot represented).

It should be noted that the signal Sn is in fact the signal to theminimized.

Under these conditions, the signal Sn is re-injected into the input 15of the filtering means which adapt the coefficients of the filter W tocalculate the signal An at the instant t+1.

The minimization operation is repeated for all the samples of thesignals Rn and Sn acquired according to the invention.

It is relevant to note that the active attenuation device according tothe invention constitutes simplified feed-forward filtering in which thesource of counter-noise does not possess any mechanical or acousticactuator.

Here, dispensing with the mechanical or acoustic actuator of the sourceof counter-noise simplifies the calculations insofar as the transferfunction is now equal to the path between the output 17 of the processorand up to the input 15 of the said processor, via the restitution block32, the summator means 6 and the acquisition block 36. This path istherefore made up solely of electronic elements in which the propagationof the electrical signal is very fast relative to the propagation of thesound waves. So short a transfer function results in a considerablesaving of calculation time as compared with feed-forward filtering witha source of counter-noise equipped with a mechanical or acousticactuator.

With reference to FIG. 3, a variant of the architecture of the deviceaccording to the invention is represented, in which the summator of thesignals Mn and An is now digital, instead of being analog as withreference to FIGS. 1 and 2. Under these conditions, digital processingmeans 100 are provided which carry out the functions of the digitalsummator means 6 and of the adaptive filtering means 12 describedearlier.

In practice, the digital processing means possess a first input 4 whichcorresponds to the input 4 of the summator means which were describedwith reference to FIGS. 1 and 2, a second input 14 which corresponds tothe first input 14 of the filtering means 12 which were described withreference to FIGS. 1 and 2, a third input 15 which corresponds to thesecond input 15 of the filtering means 12 which were described withreference to FIGS. 1 and 2, and an output 17 which corresponds to theoutput 17 of the filtering means 12 which were described with referenceto FIGS. 1 and 2.

It is relevant to note that here the third input 15 is virtual in thecase of the digital summator, since the digital value of the signal Sn,corresponding to the sum of the signals Mn and An, is known to theprocessor.

In a manner substantially similar to the analog version, the digitaldevice furthermore comprises an acquisition stage comprising thirdanalog/digital acquisition means 118, possessing an input 120 linked tothe first sensor means 2 and an output 122 linked to the first input 4of the digital processing means 100; and fourth analog/digitalacquisition means 136, possessing an input 138 linked to the secondsensor means 16 and an output 140 linked to the second input 14 of thedigital processing means 100.

The signals Mn and Rn are both applied to the digital processor 100, viathe respective acquisition modules 118 and 136. The output 17 of theprocessor 100, which delivers the digital signal Sn, is applied to theinput 130 of a restitution block 132 which restores, in analog fashion,the signal Sn to a useful-signal processing device, for example arecorder block 150.

The function carried out by the processor 100 is represented withreference to FIG. 4, as regards the summing of the signals Mn and An.

This operation is carried out by digital summator means 6 contained inthe DSP processor. The processing performed by the processor 100consists in forming the sum of the signal Mn applied to the input 4 ofthe summator means 6 and the signal An applied to the input 8 of thesaid summator means 6. The output 10 of the summator means 6 deliversthe signal Sn corresponding to the cleaned signal. The output 10 islinked to the input 15 of the adaptive filtering means in order tore-injecting the signal Sn so as to calculate the signal An at theinstant t+1. The signal An is delivered by the output 17 of thefiltering means so as to be applied to the input 8 of the summators 6.

As described earlier with reference to FIGS. 1 and 2, the processoraccording to the invention determines the coefficients of the adaptivefilter W so as to generate a signal An which, when added to the signalMn, will attenuate the components relating to the nuisance signal PNuntil the energy of the signal Sn corresponds substantially to theenergy of the useful signal Un.

In practice, the digital signal Sn from the processor 100 is transformedinto an analog signal by a digital/analog restitution module 132 similarto that described with reference to FIG. 1.

With digital summators, dispensing with the acoustic or mechanicalactuators of the source of counter-noise still further simplifies thecalculations insofar as the transfer function is now equal to that ofthe electronic path between the input 15 and the output 17 in theprocessor, that is to say equal to 1.

It is relevant to remark that the processing minimizes only thosecomponents related to the nuisance signal Pn. Consequently, theoverlapping of the energy bands of the signals Pn and Un is notproblematic.

Unlike in the case of the analog device described with reference toFIGS. 1 and 2, the signal Mn is digitized. Under these conditions, it isnecessary to adapt the sampling frequency of the processing to theuseful signal, and of the nuisance signal. If it is desired not to loseinformation related to the useful signal Un, it is expedient to work atfairly high sampling frequencies, of the order of 40 to 50 kHz, forexample, in order to cover the entire audio band stretching from 20 Hzto 20 kHz. Under these conditions, it is necessary to use processors andconverters which are very fast to perform the acquisitions andcalculations.

According to another embodiment of the invention, in respect of theanalog or digital version, there is provision to add automatic gaincontrol to the reference channel, that is to say that associated withthe sensor 16.

This controller can replace or supplement the pre-amplification in theanalog/digital acquisition module associated with the acquisition of thereference acoustic signal.

The function of this controller is to provide the reference signal withthe same amplitude, irrespective of the amplitude of the input signal.

Thus, in the presence of strong or weak nuisance noise, the processorprocesses reference noise of like amplitude. This results in a betterdynamic range in respect of the signal processing.

The subject of the present invention is also a process for extracting auseful acoustic signal Un from a composite acoustic signal Mn comprisingnuisance components Pn.

Generally, the process comprises the following steps:

a) providing first sensor means 2, which are arranged at a first chosenlocation and are able to pick up the composite acoustic signal Mn,

b) providing second sensor means 16, which are arranged at a secondlocation chosen according to a predetermined geometric relationship withthe first location, and are able to pick up, in real time, a referenceacoustic signal Rn which it substantially correlated with the nuisancecomponents Pn of the composite acoustic signal Mn and is capable ofpropagating from the said second location to the said first location,

c) providing filtering means 12 possessing at least a first input 14linked to the second sensor means 16, and an output 17, and

d) providing extraction means 6 possessing a first input 4 linked to thefirst sensor means 2, a second input 8 linked to the output 17 of thefiltering means 12, and an output 10, and

e) determining at least some of the coefficients of the filtering means12 until the signal Sn leaving the output 10 of the extraction meanscorresponds substantially to the useful acoustic signal Un selectivelyrid of the nuisance components Pn.

Usually, the filtering means 12 are of adaptive type with a second input15 linked to the output 10 of the extraction means 6, like thosedescribed with reference to FIGS. 1 to 4.

Under these conditions, step a) according to the invention consists inthe following step:

e1) adapting at least some of the coefficients of the filtering means12, in real time, according to an algorithm chosen so as to minimize theenergy of the composite acoustic signal Mn as a function of the energyof the reference acoustic signal Rn, until the signal Sn leaving theoutput 10 of the extraction means 6 corresponds substantially to theuseful acoustic signal Un selectively rid of the nuisance components Pn.For certain useful acoustic signal extraction applications, it may bebeneficial and cheaper to Use fixed, preferably analog, filtering means,the filtering coefficients of which are determined, beforehand oncompletion of step e1), with the aid of adaptive filtering means such asthose described with reference to FIGS. 1 to 4. The conditions ofpropagation of the acoustic signals in extraction mode with fixedfiltering means must then be substantially similar to those in the modefor determining the filtering coefficients with the adaptive filteringmeans.

We claim:
 1. A device for extracting a useful acoustic signal (Un) froma composite acoustic signal (Mn) comprising nuisance components (Pn),the device being one which comprises:first sensor means (2), which arearranged at a first chosen location and are able to pick up thecomposite acoustic signal (Mn), second sensor means (16), which arearranged at a second location chosen according to a predeterminedgeometric relationship with the first location, and are able to pick up,in real time, a reference acoustic signal (Rn) which is substantiallycorrelated with the nuisance components (Pn) of the composite acousticsignal (Mn) and is capable of propagating from the said second locationto the said first location, filtering means, of the adaptive type, (12)possessing a first input (14) linked to the second sensor means (16), asecond input (15) and an output (17), and extraction means (6)possessing a first input (4) linked to the first sensor means (2), asecond input (8) linked to the output (17) of the filtering means (12),and an output (10) linked to the second input (15) of the filteringmeans (12), wherein the second location is chosen so that the referenceacoustic signal (Rn) contains no information related to the usefulsignal (Un) and wherein the filtering means (12) are able to minimizethe energy of the composite acoustic signal (Mn) as a function of theenergy of the reference acoustic signal (Rn), by adapting at least someof the filtering coefficients in real time, until the signal (Sn)leaving the output (10) of the extraction means (6) correspondssubstantially to the useful acoustic signal (Un) selectively rid of thenuisance components (Pn).
 2. The device as claimed in claim 1, whereinthe extraction means (6) are summator means of the analog type,whereinit furthermore comprises an acquisition stage comprising firstanalog/digital acquisition means (18), possessing an input (20) linkedto the second sensor means (16) and an output (22) linked to the firstinput of the adaptive filtering means (12); and second analog/digitalacquisition means (36), possessing an input (38) linked to the output(10) of the analog summator means (6) and an output (40) linked to thesecond input (15) of the adaptive filtering means (12), and wherein itfurthermore comprises a restitution stage comprising firstdigital/analog restitution means (32), possessing an input (30) linkedto the output (17) of the adaptive filtering means (12) and an output(34) which delivers a signal (An) applied to the second input (8) of theanalog summator means (6).
 3. The device as claimed in claim 1, whereinthe extraction means (6) are summator means of the digital type;whereinit comprises digital processing means (100) possessing first (4), second(14) and third (15) inputs, and an output (10), the said digitalprocessing means (100) being able to carry out the functions of thedigital summator means (6) and of the adaptive filtering means (12),wherein it furthermore comprises an acquisition stage comprising thirdanalog/digital acquisition means (118), possessing an input (120) linkedto the first sensor means (2) and an output (122) linked to the firstinput (4) of the digital processing means (100); and fourthanalog/digital acquisition means (136), possessing an input (138) linkedto the second sensor means (16) and an output (140) linked to the secondinput (14) of the digital processing means (100).
 4. The device asclaimed in claim 3, wherein it furthermore comprises a restitution stagecomprising second digital/analog restitution means (132) possessing aninput (130), linked to the output (10) of the digital processing means(100), and an output (134).
 5. The device as claimed in claim 1, whereinthe digital filtering means (12) are of the digital signal processortype and wherein the minimization algorithm is of the least mean squarestype.
 6. A process for extracting a useful acoustic signal (Un) from acomposite acoustic signal (Mn) comprising nuisance components (Pn), theprocess comprising following steps:a) providing first sensor means (2),which are arranged at a first chosen location and are able to pick upthe composite acoustic signal (Mn), b) providing second sensor means(16), which are arranged at a second location chosen according to apredetermined geometric relationship with the first location, and areable to pick up, in real time, a reference acoustic signal (Rn) which issubstantially correlated with the nuisance components (Pn) of thecomposite acoustic signal (Mn) and is capable of propagating from thesaid second location to the said first location, c) providing filteringmeans (12), of the adaptive type, possessing a first input (14) linkedto the second sensor means (16), a second input (15) and an output (17),and d) providing extraction means (6) possessing a first input (4)linked to the first sensor means (2), a second input (8) linked to theoutput (17) of the filtering means (12), and an output (10), the saidprocess being one wherein the second location is chosen so that thereference signal (Rn) contains no information related to the usefulsignal (Un), and wherein it comprises a step consisting in: e)minimizing the energy of the composite acoustic signal (Mn) as afunction of the energy of the reference acoustic signal (Rn), byadapting at least some of the filtering coefficients in real time untilthe signal (Sn) leaving the output (10) of the extraction means (6)corresponds substantially to the useful acoustic signal (Un) selectivelyrid of the nuisance components (Pn).